Microelectromechanical systems (“MEMS”) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have structure suspended above a substrate, and associated electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
The associated electronics, substrate, and movable structure typically are formed on one or more dies (referred to herein simply as a “die”) that are secured within a package. The package includes interconnects that permit the electronics to transmit the movement data to the external devices. To secure the die to the package interior 18, the bottom surface of the die commonly is bonded (e.g., with an adhesive or solder) to an internal surface of the package. Accordingly, in such case, substantially all of the area of the die bottom surface is bonded to the internal surface of the package.
MEMS inertial sensors/die are sensitive to environmental factors, such as disparate material expansion between the die and its package. Specifically, this disparate expansion commonly is caused by mismatched coefficients of thermal expansion (“CTE”) between the materials forming the die bottom surface and the internal package surface that secures the die. In fact, these CTE mismatches can cause the sensor to deliver incorrect motion measurements. For example, when implemented as an accelerometer within an automobile airbag system or as a gyroscope in an automobile traction control system, CTE mismatches can produce results that can cause the automobile to operate erratically. Consequently, such incorrect measurements can lead to bodily injury or death for drivers, their passengers, or others near the moving automobile (e.g., people in other automobiles).
To reduce this problem, MEMS inertial sensors commonly are secured within ceramic packages rather than within two other well known, less costly, but widely used package types; namely, “transfer molded” packages and “premolded” packages. Specifically, both transfer molded packages and premolded packages have a copper leadframe to which the die is secured. The CTE difference between copper and silicon (i.e., the material forming the die), however, is much greater than the CTE difference between ceramic and silicon.
Moreover, transfer molded and premolded packages also typically cannot provide hermeticity. Accordingly, if such types of packages are used with certain MEMS sensors, additional processes are required to ensure that moisture does not affect the sensor itself.
For these and other reasons, those in the art are motivated to use ceramic packages for MEMS inertial sensors rather than the other two types of packages. Undesirably, ceramic packages typically are more expensive than the other two types of packages. In addition, securing a MEMS sensor die within a ceramic package requires a larger number of process steps (when compared to the other noted types of packages), thus further increasing production costs. In fact, in many MEMS sensor applications using ceramic packages, the packaging cost far exceeds the cost of producing the MEMS sensor itself.